Thin films of functionalized single-wall carbon nanotubes were deposited on silicon chips by drop-coating and inkjet printing. These sensors were subjected to 1-100 ppm NOx, CO, H2S and H2O vapor in synthetic air. We have found that besides the expected changes in the electrical resistance of the film, there are also characterteristic differences in the noise pattern of the resistance vs. time function. This phenomenon is called fluctuation enhanced sensing and it can be used to increase the amount of information gathered from a carbon nanotube sensor device. The main advantage of fluctuation enhanced sensing is the improved selectivity of the sensor even if changes in electical resistance are rather low. Combined with differentiation based on modifying the adsorption characterstics of the nanotubes (e.g. by covalent functionalization), fluctuation enhanced sensing appears to be a very useful method for bringing cheap and reliable carbon nanotube based chemical sensors to the market.
Interaction of target molecules with the evanescent wave of light guided in optical fibers is among the most promising sensing schemes for building up smart chemical sensor technologies. If the technique of optical time domain reflectometry (OTDR) is combined with silicone-clad quartz glass fibers distributed chemical sensing is possible. Hydrocarbon (HC) detection and location is done by automated identification of the position of the corresponding step drop (light loss) in the backscatter signal induced by local refractive index increase in the silicone cladding due to a penetrating HC compound. A commercially available mini-OTDR was adapted to sensing fibers of up to nearly 2-kilometer length and location of typical HC fuels could be demonstrated. The instrument is applicable for fuel leakage monitoring in large technical installations such as tanks or pipelines with spatial resolution down to 1 m. A similar technique using measurements in the Vis spectral range is being developed for health monitoring of large structures, e.g., for early detection of corrosion caused by water ingress and pH changes in reinforced concrete. Here, a pH indicator dye and a phase transfer reagent are immobilized in the originally hydrophobic fiber cladding, leading to a pH induced absorption increase and a step drop signal in the backscatter curve. The configuration of the distributed sensing cables, the instrumental setups, and examples for HC and pH sensing are presented.
The complete knowledge of the intrinsic electronic properties of new materials like fullerenes is essential for their technical application in optoelectronic and photonic devices, as well as in terms of the fundamental physical processes. The value of the intrinsic energy gap, the shape of the bandedge, and the nature and origin of the radiative recombination channels after photoexcitation of C60 are still unsettled. We report on photoluminescence emission and excitation spectroscopy on high-quality C60 single crystals and high-quality C60 thin films grown by molecular beam epitaxy (MBE) on mica substrates.
The observation of a broadband electroluminescent emission from fullerene crystals, with a spectral distribution comparable to that of the photoluminescence at high excitation densities is described. The emission intensity is highly nonlinearly dependent on the current. The response of the crystal to the application of an alternating current is investigated to determine the dynamic parameters of the emission process. In particular the frequency dependence of the emission intensity is described. Since the observed high frequency cut-off behavior cannot be mimicked by a simple equivalent circuit, a rate equation model is used to describe the state of the system. Fits of the model to the observed behavior provide rate constants which compare favorably to those reported for excited state decay in fullerenes. Observation of electroluminescence requires crystals of comparatively high conductivity and it is noted that the conductivities of different crystals from the same batch can vary by many orders of magnitude. Furthermore, when driven by high currents, the electroluminescent crystals undergo an irreversible increase in this conductance. In the region of the irreversibility, the electroluminescence output becomes unstable, and the current threshold for the onset of emission increases in a sample which has been driven to progressively higher currents increases. The process is discussed in terms of a current driven increase in the active volume of highly conducting crystalline pathways in the crystal.
Picosecond time resolved photoluminescence and photoconductivity measurements are performed to investigate the influence of high intensity illumination on the properties of Fullerene crystals. A highly nonlinear dependence of both the photoluminescence characteristics and the photoconductive response of the fullerenes is seen and temperature dependent measurements indicate that the nonlinear processes are associated with an insulator- metal phase transition in the material, and thus that the electronic properties of the excited state are dramatically altered at high excited state densities. Application of a simple phenomenological model to calculate the contribution of exchange and correlation energies supports the feasibility of such an interpretation. A further manifestation of this behavior is the emergence of a broadband electroluminescent emission above a critical injection current density.
Conference Committee Involvement (2)
Carbon Nanotubes, Graphene, and Associated Devices II
5 August 2009 | San Diego, California, United States
Carbon Nanotubes and Associated Devices
10 August 2008 | San Diego, California, United States